xen init script: don't fail when being run in domU

xen init script: don't fail when being run in domU

When installing xen-utils-V in a driver domain domU, it drags in
xen-utils-common, which also contains the init script for xenstored and
xenconsoled.

Installing the package will fail right away, because it exits non-zero
after checking whether it's running in a xen dom0 or not:

 systemd[1]: Starting LSB: Xen daemons...
 xen[7215]: Starting Xen daemons: (warning).
 systemd[1]: xen.service: Control process exited, code=exited, status=255/EXCEPTION
 systemd[1]: xen.service: Failed with result 'exit-code'.
 systemd[1]: Failed to start LSB: Xen daemons.
 dpkg: error processing package xen-utils-common (--configure):
  installed xen-utils-common package post-installation script subprocess returned error exit status 1

Since there's nothing to be fixed, there should not be a warning. It's
totally fine to skip xenstored, xenconsoled and qemu steps in this case,
so just exit cleanly.

Link: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=922033
Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Signed-off-by: Hans van Kranenburg <hans@knorrie.org>
Acked-by: Ian Jackson <ijackson@chiark.greenend.org.uk>